Current techniques for fabrication of a resistive element to be used in electronic devices, e.g., hybrid IC and thermal heads, include thick film formation and thin film formation. The former process comprises coating a resistive element-forming paste, for example, a dispersion of ruthenium oxide powder and glass frit in organic vehicles consisting of a solvent and a resin, on a substrate by screen printing followed by calcination to form a resistive element as disclosed, e.g., in JP-A-53-100496 and JP-A-54-119695 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The latter process is application of vacuum technology and comprises depositing a thin film of a sparingly soluble metal, e.g., tantalum, on a substrate by sputtering and making a pattern by photolithographic technique to form a thin resistive film as disclosed, e.g., in JP-A-55-63804. The resistive element produced by the thin film formation technique is now of use in some thermal heads.
Resistive elements to be used in electronic devices such as a hybrid IC and a thermal head have been increasingly demanded to serve for multi- and high-functions by themselves. In the case of a thermal head, for instance, there have been demands for high resolving power, improved tone reproduction, color reproduction, reduction in power consumption, and reduction in size. To meet these demands, resistive elements with a variety of characteristics, particularly with the resistivity ranging from low to high, have been demanded.
While in the above-mentioned thick film formation resistivity is controlled by a mixing ratio of ruthenium oxide conductive component and a glass component, the resulting resistive element fundamentally consists of a composite system of two oxide components each having a particle size distribution. It has been therefore difficult to sufficiently control the resistivity simply by adjustment of the mixing ratio.
In the thin film formation, vacuum evaporation or sputtering is conducted by using an evaporation source (target) of prescribed composition prepared according to a desired resistivity or by using a plurality of evaporation sources to form a deposit having a desired resistivity. For production of a resistive element whose resistivity widely ranges, a number of targets are required and, also, the vacuum evaporation or sputtering must be under very strict control so as to maintain the composition of the deposit always constant.